Staggered field effect on the one-dimensional S=1/2 antiferromagnet Yb4As3

Inelastic neutron scattering measurements of magnetic excitations in the ch arge-ordered state of Yb4As3 have been performed under magnetic field up to about 6 T. By applying a magnetic field, the spectrum at the one-dimension al wave vector q = 1 [ pi /d] changes drastically from a broad one correspo nding to the spinon excitation continuum of the one-dimensional S = 1/2 spi n system to a sharp one at a finite energy, indicating the opening of an en ergy gap in the system. The magnetic field dependence of the gap is well fi tted by the power law H-2/3. The experimental result gives strong evidence for the existence of a staggered field alternating along Yb3+ chains induce d by the Dzyaloshinsky-Moriya interaction.